A voice over a loudspeaker said: "We ask you to remember all those whose lives have been lost in the tragic events in America."

Queen Elizabeth II was among those attending a memorial service at the cathedral at 12 noon today.

In Lockerbie, Scotland, where 270 died when a plane was blown out of the sky in December 1988, the town hall bell tolled to signal the start of the three-minute silence. Two police officers stood at the town's war memorial bowing their heads.

In Wales, hundreds gathered in the center of Cardiff in silent tribute to those killed in the U.S. tragedy.

Workers poured out of local offices to join Wales' political leaders for the three-minute silence at the war memorial outside the Assembly's civil service headquarters in the city centre.

Across London, offices, shops, courts and factories fell silent, as did the Houses of Parliament -- meeting in a rare special session to debate Britain's response to the attacks.

Tourist attractions such as the giant London Eye Ferris wheel along the River Thames also came to a stop.

Pubs were asked to delay their 11 a.m. opening by five minutes, shop staff were told not to process transactions, and black cabs and buses pulled over for three minutes.

About 100 British nationals were among those killed in the terrorist attacks that destroyed New York's World Trade Center twin towers, UK officials said, adding that the number could reach the "middle hundreds."

Elsewhere, Ukraine also observed three minutes of silence. Ukrainians also lined up to write in a book of condolences at the embassy in the capital Kiev.

"It's awful. What happened is not only the pain of the U.S., it's the world's pain," wrote Ira Timlina.

Ukrainian state television interrupted a movie to announce a nationwide day of mourning.

Germany held a nationwide five minutes of silence, and hundreds gathered in front of the U.S. Embassy in Berlin amid a sea of flowers spread along the street leading to the building.

"I am here to show that the German people feel for the American people," said 37-year-old Berndt Mattigk. "I am sad and horrified that the American people have suffered an act of war on their soil."

Finnish radio stations stopped broadcasting at noon and more than 400 trams and buses in Helsinki stopped for one minute of silence.

"Innocent people were killed, and we showed our respect for them and their families," said tram driver Mika Savela, 31.

Copenhagen city buses also stopped for one minute at noon as part of a nationwide display of "compassion for the victims, their families and the American people," said Mads Lebech, chairman of the city bus company.

In Austria, church bells rang for three minutes then fell quiet as people joined in three minutes of silence.

Earlier on Friday, the 15 government leaders of the EU vowed to chase down those responsible for the attacks on the United States and hold accountable any countries providing them shelter.

The EU leaders signed a statement outlining an anti-terrorism policy.

"We will not, under any circumstances, allow those responsible to find refuge, wherever they may be," the statement said. "Those responsible for hiding, supporting or harbouring the perpetrators, organizers and sponsors of these acts will be held accountable."

The EU nations also said they would unify their arrest warrants and extradition orders and bolster their common foreign and security policies in their fight against terrorism.

EU transport ministers are holding an emergency meeting Friday to discuss airport security improvements. EU foreign ministers earlier expressed full solidarity with the U.S., declaring Friday's three-minute silence as part of a day of mourning.

The declaration was expanded to include all 43 member states of the Council of Europe based in Strasbourg, France.